Topics:

Magnetic Resonance Imaging of the Abdomen: Applications in the Oncology Patient

Magnetic Resonance Imaging of the Abdomen: Applications in the Oncology Patient

ABSTRACT: Cross-sectional imaging of the abdomen in oncology patients presents unique challenges and opportunities. A close working relationship between the oncologist and radiologist is essential for the exchange of the clinical and imaging information necessary for optimizing patient diagnosis and management. Compared to helical computed tomography (CT), magnetic resonance imaging (MRI) of the abdomen and pelvis offers important advantages, including superior soft-tissue contrast. The multiplanar capabilities of MRI allow for direct coronal or sagittal imaging, providing a truer anatomic presentation of abdominal and pelvic masses. Recent advances in MRI, including the use of intravenous (IV) and oral contrast agents, the development of high-performance imagers, and improved surface coil designs, facilitate more rapid abdominal imaging with superior image quality. All of these features combine to produce a versatile imaging examination with exquisite sensitivity for depicting abdominal and pelvic tumors. In this article, we will review the clinical applications for hepatic and extrahepatic abdominal MRI in the oncology patient. The MRI techniques and protocols described can be applied to most commercially available high-field magnetic resonance imagers. [ONCOLOGY 14(Suppl 3):5-14, 2000]

The imaging evaluation of oncology patients
requires accurate depiction and characterization of all hepatic and
extrahepatic tumors. While helical computed tomography (CT) has been
the workhorse of most radiology departments, recent advances in
abdominal magnetic resonance imaging (MRI) have moved it to the
forefront of oncologic imaging at our institution, the Sharp Memorial Hospital.[1,2]

Compared to helical CT, MRI of the abdomen offers the potential
advantages of superior soft-tissue contrast and multiplanar imaging.
The extent to which we are able to distinguish tumor from normal
abdominal soft tissues is central to our ability to accurately depict
the dimensions of a tumor. Magnetic resonance images display a much
wider range of soft-tissue contrast, making tumor masses easier to
distinguish from adjacent soft tissues (Figure
1
). The addition of contrast agents provides differential
enhancement of tumor and normal soft tissues, thus further improving
the delineation of hepatic and extrahepatic tumor (Figure
2
).

Among the recent advances in MRI are faster pulse sequences,
breath-hold imaging, and use of intravenous contrast agents and
surface coils, all of which have improved image quality and shortened
examination times. The versatility of abdominal MRI is unmatched by
any other imaging examination. For these reasons, at our institution,
MRI has evolved from a problem-solving study to become the primary
imaging examination in many patients with malignancy. In this review,
we will discuss the spectrum of oncologic applications for abdominal
MRI and will highlight areas in which newer MRI techniques offer
significant advantages over helical CT.

Hepatic MRI

Overview

The depiction of a focal liver lesion requires a difference in signal
intensity between the lesion and the adjacent liver parenchyma. The
lesion may be either more or less intense than the surrounding liver.
Most liver masses are easily depicted on unenhanced T1-weighted or
T2-weighted MRIs (Figure 3).
However, some tumors produce minimal changes in T1- and T2-relaxation
and, therefore, show limited contrast with the surrounding liver.

Contrast agents are used to accentuate the inherent differences in
liver-lesion signal intensity. These contrast agents facilitate
differential enhancement of liver parenchyma and masses, as well as
promote lesion depiction and, to some degree, lesion characterization (Figure
4
).[3,4]

Gadolinium chelates were the first intravenous contrast agents to be
approved. These nonspecific extracellular agents rapidly equilibrate
from the intravascular space into the extracellular space after
injection. The use of gadolinium chelates has become an integral part
of MRI of the liver and extrahepatic abdomen. Although liver-specific
contrast agents are now available, gadolinium chelates continue to
offer significant advantages in abdominal MRI.

Gadolinium chelates uniquely provide important information about
tumor perfusion, which is key in the assessment of liver masses. They
assist with the detection and characterization of liver lesions and
in the establishment of the volume of viable perfused tumor.

Gadolinium chelates are equally important for MRI of the extrahepatic
abdomen. The interstitial accumulation of these agents within
peritoneal, omental, and gastrointestinal tumor produces marked
enhancement and is crucial to accurate tumor staging. Depiction of
lesions within solid visceral organs, such as the pancreas, kidneys,
and spleen, also improves following gadolinium injection.

MRI Technique

For effective hepatic MRI, we combine unenhanced axial T1-weighted
spoiled gradient recalled echo (SPGR), and fat suppressed T2-weighted
MRI with rapid, serial dynamic gadolinium-enhanced SPGR MRI. Three
sets of dynamic gadolinium-enhanced liver images are obtained during
the arterial, portal-venous, and equilibrium phases of liver
enhancement. On high-field strength systems, rapid SPGR T1-weighted
sequences are obtained after a 0.1-mmol/kg bolus injection of
gadolinium chelate. By imaging the entire liver volume during a short
period of suspended respiration, motion artifact is eliminated, while
the intravascular gadolinium provides important information on tumor perfusion.

Evaluation of Hepatic Metastases

Compared to CT with iodinated contrast material, MRI has proven to be
superior in evaluating metastatic liver disease.[5-9] Semelka et
al[7] compared single-phase helical CT and MRI with fat suppressed
T2-weighted and gadolinium-enhanced SPGR in 89 patients with focal
liver masses. In 49% of patients, MRI detected more lesions than did
helical CT, with MRI depicting 519 true-positive liver masses vs 295
lesions depicted with helical CT.

Delineation of lesion borders is often easier on MRI, allowing for
more consistent measurement of hepatic metastasis (Figure
4
).

Hypervascular Hepatic Metastases

Hypervascular metastases from renal cell carcinoma, pancreatic islet
cell tumors, breast carcinoma, thyroid carcinoma, sarcomas, and
carcinoid tumors are supplied by the hepatic artery and enhance
rapidly following the injection of gadolinium chelates. Since 75% to
80% of the blood supply to the liver derives from the portal vein,
there is only minimal liver enhancement on these early images. Thus,
on arterial phase images, hypervascular metastases will show marked
enhancement against a background of minimally enhancing liver
parenchyma (Figure 5).[8]

Hypovascular Hepatic Metastases

Hypovascular metastases arise from colon carcinoma, pancreatic
carcinoma, transitional cell carcinoma, and lung cancer and are best
depicted on portal-venous phase gadolinium-enhanced images. These
metastases receive a minimal supply of blood from the hepatic artery
or portal vein. During the portal-venous phase, the liver parenchyma
demonstrates marked enhancement while the hypovascular metastases
show minimal enhancement, producing the greatest difference in
liver-lesion signal intensity (Figure
6
).[6,7,9] Gradual peripheral enhancement and heterogenous
central enhancement of these lesions occurs on later images.

On arterial phase images, hypovascular metastases are poorly
visualized, appearing only vaguely distinct from the adjacent liver.
Since neither the tumor nor the liver is enhancing at this point,
these hypovascular lesions may not be depicted. However, due to the
high-contrast resolution of MRI, it is not uncommon to see thin
peripheral enhancement of hypovascular metastases on
gadolinium-enhanced capillary phase images. On equilibrium phase
gadolinium-enhanced images, hypovascular metastases will become
indistinct due to nonspecific interstitial accumulation of the
contrast material. The pattern of peripheral washout of the contrast
may also be noted.

Evaluation of Hepatocellular Carcinoma

Hepatocellular carcinomas (HCC) manifest a highly variable appearance
on unenhanced T1-weighted and T2-weighted images. On T1-weighted
imaging, HCC are most often hypointense compared with normal liver,
although hyperintense lesions or areas of hyperintensity within a
hypointense HCC can be seen. These hyperintense regions within the
HCC reflect the presence of fat, copper, or protein. On T2-weighted
imaging, HCC are generally hyperintense, although well-differentiated
HCC may be isointense compared with liver parenchyma, thus limiting
their detection.

Due to the variable appearance of HCC on T1-weighted and T2-weighted
MRIs, dynamic gadolinium-enhanced imaging can play an important role
in the diagnosis of this primary liver tumor.[10,11] Compared to
helical CT with iodinated contrast material, the superior contrast
resolution of MRI facilitates detection of small enhancing HCC on
arterial phase gadolinium-enhanced SPGR images. In the setting of
cirrhosis, any abnormal foci on arterial phase images should be
considered highly suggestive of a developing HCC. In a recent study,
Yamashita et al found that arterial phase gadolinium-enhanced MRI is
superior to helical CT for detecting hepatocellular carcinoma.[10]

Following IV injection of gadolinium, the pattern and degree of
enhancement of HCC is related to tumor differentiation and histologic
subtype. Well-differentiated tumors often show minimal arterial phase
enhancement that washes out on portal-venous phase images, leaving
the tumor hypointense compared with the adjacent liver. The presence
of a capsule will be indicated by a hypointense rim on early images
that enhances on delayed images.

Moderately or poorly differentiated HCC are characterized by dilated
sinusoidal spaces, which produce moderate or marked enhancement with
gadolinium on arterial phase images. Enhancing tumor nodules are
often best depicted on these arterial phase dynamic images.
Portal-venous invasion by an HCC is most accurately identified on
gadolinium-enhanced breath-hold SPGR images. Expansion of the portal
vein and replacement of the intravascular gadolinium by tumor
thrombus is evidence of portal-venous tumor extension.

Pages

 
Loading comments...
Please Wait 20 seconds or click here to close